248 
Proceedings of the Iioyal Society 
ment, which has been since that time in his possession, and is 
now printed verbatim : — 
“ April 10 thy 1873 . 
“ Carbonic Acid — partly liquid, partly gaseous — fills the cavity. 
“ Distillation, when one end is heated ever so slightly above the 
other, the circumstances being of almost unexampled favourability 
for such an effect. Hence the apparent motion of the bubble. 
It is not the same bubble as it moves. 
“ General problem suggested by this, and easily solved by the 
dynamical theory of heat. 
“ Find distribution of Least Entropy of contents of a vessel 
where the temperature is a given function of the position in space, 
and the contents are one or more substances (say, for simplicity, 
not chemically acting on one another) in two or more different 
states (as to latent heat, &c.) 
“ This is more (much more) than the whole affair. 
P. G. Tait.” 
A day or two afterwards I tried the experiment on a large scale, 
with the assistance of my laboratory students, and at once 
succeeded in showing to them, and to several of my colleagues, Mr 
Sang’s results in quill tubes of three or four inches in length, 
containing sulphurous acid partly in the liquid and partly in the 
gaseous state. 
The present communication, like that of Professor Swan which 
follows it, is now made to the Society at the request of Mr Sang 
himself. 
(2.) Professor Swan. 
The following note is a narrative of experiments made by me 
nine months ago, on the 5th and 6th May 1873, on the motions 
observed in the cavities of Iceland spar by Mr Sang, with an 
explanation of the manner in which I believe these singular move- 
ments to be caused by heat. Being unwilling to interfere with 
Mr Sang’s investigations then in progress, I did not at the time 
seek to publish my note, but forwarded it in a sealed envelope to 
the secretary of the Society, in whose custody it has since remained. 
It is now communicated to the Society in accordance with Mr 
Sang’s wishes, and is printed without alteration or addition. 
